1. Field of the Invention
This invention relates generally to the production of titanium dioxide on a porous catalyst carrier and more particularly to the control of the polymorphic form of titanium dioxide on said carrier.
2. Prior Art
Titanium dioxide can exist in three polymorphic forms: anatase, brookite and rutile. Brookite is found rarely as natural single crystals and has been prepared synthetically. Anatase and rutile appear naturally and are also produced commercially. In commercial production the formation of the particular crystal species is controlled by the manufacturing process. Anatase and rutile can best be distinguished by X-ray diffraction as known to those skilled in the art.
Titanium dioxide has been used as a carrier, a coating for a carrier, or a co-catalyst for oxidation and as a catalyst for other reactions. Differences have appeared concerning the desirability of the anatase or rutile form for such reactions.
The titanium dioxide is generally prepared commercially by hydrolytic precipitation from solutions of titanium sulfate or vapor phase oxidation of titanium tetrachloride. Normally, hydrolysis from sulfate solutions yields the anatase form and hydrolysis from chloride solutions (sulfate free) yields the rutile form. These normal products can be reversed in either case by special techniques such as, for example, seeding with nuclei of the other form or by addition of chemical additives. However, the commercial products generally have crystal sizes too large to permit impregnation of usual porous carriers.
Titanium tetrachloride is preferred as the starting material for the production of titanium dioxide in accordance with this invention because if titanium sulfate is the starting material, the sulfuric acid by-product would need to be removed by washing while the HCl by-product of titanium tetrachloride hydrolysis is volatile. The saving of one step in preparation with no possible sulfate contamination seems desirable. The general overall reaction is as follows: EQU TiCl.sub.4 +2H.sub.2 O.fwdarw.TiO.sub.2 .dwnarw.+4HCl.uparw.
A porous carrier in a catalytic system is a relatively inert material that serves as a support for the active catalyst. The functions of a carrier with respect to any catalyst, however, varies with the individual case. The carrier in many instances has an extremely great influence upon the activity and selectivity of the catalytic salts which it supports.
In a fluid bed catalytic system, as an example, the carrier plays several roles. The carrier must have the properties of good fluidization and heat-transfer after the catalytically active salts are added to the carrier. The characteristics of the carrier may also affect either the chemistry of the active salt disposition or the activity and selectivity of the final catalyst in an unknown fashion. Thus an a priori method of carrier selection is not available for these latter effects. The carrier in accordance with this invention is impregnated with the titanium dioxide and, if desired, can be further impregnated with the other catalytic salts.
The making of carriers that include crystalline titanium dioxide in the anatase form for oxidation catalysts, either alone or as a co-catalyst, is known. Generally, the processes for making TiO.sub.2 carriers have depended upon the titanium dioxide being in the desired crystalline form as the initial starting material. U.S. Pat. No. 3,565,919, however, discloses that impregnating a carrier with an aqueous methanol solution of titanium tetrachloride and heating the product in the presence of oxygen at 130.degree. C. to 600.degree. C. thereby converted the titanium compound into the anatase form, and when a rutile/anatase form was desired as in Example I of the patent, titanium dioxide in the rutile form was used as the carrier. In working with xerogels made from various aqueous alcoholic titanium tetrachloride solutions, M. Vallet Regi et al. reported, in Anales De Quimica, Vol. 76, pages 172 et seq., that the gels which are prepared from titanium alcoholates are amorphous at room temperature, start to transform to anatase at 200.degree. C. and to the rutile form at 600.degree. C. and the specific alcohol being used to prepare the xerogel seemingly influences the gel's behaviour during thermal decomposition.